Writer-director Jim Mickle’s adaptation of Joe R. Lansdale’s pulp novel is a genre picture that can’t settle on a genre — which is what makes it so entertaining. “Cold In July” starts out as a suspense picture, with Michael C. Hall playing a small-town Texan named Richard who shoots and kills a home invader and then waits nervously for the burglar’s ex-con father (played by Sam Shepard) to retaliate. Then the movie becomes a mystery, when the local police force resolves the case in a way that Richard finds unsatisfactory. Finally, “Cold In July” becomes a bloody action film, when Richard teams with a colorful private eye (Don Johnson) to confront the Dixie Mafia. Not every part works — nor does it all fit together in a way that has much larger meaning — but it’s a gripper from start to finish, because of Mickle’s lean style, and because it’s impossible to know in the first five minutes where the movie will end up.